Method of producing electron emitting cathodes



July 22, 1958 j T E. F. REXER 2,844,494 I METHQD 0F PRODUCING ELECTRON EMITTING CATHODES Filed Oct. 21. 1955 PREP RE POROUS SPONGE MEI'AL MATRIX IMPREGNATE MATRIX WITH ALKALINE EARTH COMPOUND. [m AQUEOUS SOLUTION 0F B0(NO3)2 $1(NO3)2] ADD CARBONATE TO IMPREGNATED MATRIX [m AQUEOUS SOLUTION DECOMPOSE ALKALINE EARTH CARBONATE TO OXIDE ACTIVATE CATHODE INVENTOR. EDWARD E REXER ayhgzw ATm/HVEY cathode matrix;

a P atented July .22, 1958' METHOD OF PRODUCING ELECTRON EMITTING CATHODES I Edward F.'Rexer, Minneapolis, Minn., assignor to Minneapplis-Honeywell Regulator Company, Minneapolis, Mmn., ncorporation of Delaware Application October 21,1953, Serial No. 387,537

9 Claims. or. 117-413 ,7 The present invention relates to an improved electron emissive cathode having superior electrical and mechani- ..cal properties, and to the method for producing the same.

More specifically, this invention relates to an improved dispensertype cathode which includes a porous matrix impregnated throughout with an oxide of an alkaline earth metal, the porous matrix being sintered and formed to its final desired size and shape before impregnation.

According to present day methods of producing electron emitting cathodes, the emitting material, for example, barium-strontium oxide,'is sprayed onto, brushedonto,

onincchanically compressed directly into'the cathode base metal matrix. The oxide coating may be applied directly to the cathode body as such, or it may be applied in the form of a car-bonate. Although these procedures have met with general approval in the field, they leave room for considerable improvement, particularly since these 'cathodes are subject to rapid deterioration due to the tendency of the emissive coating to peel or flake away from the matrix after relatively short periods of operation, and more particularly during periods of rough handling or operation. Basically, there is insufficient adherence between the matrix and the emissive coating in these present a aciously adheres or bonds itself to the matrix body with out need for an adhesive or bonding agent, and it is able stantially continuous supply of emitting material available for the electron emitting body.

An additional object of the present invention A vide a dispenser type electron emitting cathode which has a .low interface resistance between the matrix body and the electron emitting substance, 4

Reference is made to the accompanying drawing which,

shows in block diagram form af flow sheet ofasuggested process inaccordance with the present invention;

According to the present invention alporous cathode base metal matrix is prepared-inaecordance with "conventional metallurgical procedures. For example, powdered nickel is thoroughly mixed with a volatile binder, anda slurry made therefrom. The slurry, preferably being of pasty consistency, is thenplaced in a die and pressed with sutficient pressure to compress the slurry, intothe desired form. I prefer to use relatively low compressing pressures in order that the resulting matrix body be highly .porous. The bodies are then sintered under a neutral or reducing atmosphere such as hydrogen, the binder preferably being volatilized at the sintering temperature so as to -1eave;a porous metal matrix. This matrix is. then cleaned :to remove impurities therefrom and. is-ready for impregnation. 1 When nickel isemployed as the base. metal, a matrix having a porosity of 20% to is satisfactory. Porosity is defined as P-(l 100 wherein:

P=porosity in percent 1 D =weight of metal body per volume of metal body D density of base metal, for example, nickel. 1

As a general rule, as the powder size decreasesthe ,numberof pores increases, which is desirable. For example, in a nickel powder having a size ranging from 37 to 44 microns, a body having a porosity of 27.4% has been found to be satisfactory, whereas, a powder size having a size range of under. 37 microns, pressing toa porosity of 29.2% was very satisfactory. It should be noted at this point, however, that a relatively high degree of porosity is desiredin the pellet matrix since this creates a body wherein, a substantial amount of the inner pores are accessible from the outer surfaces of the body; This is based upon the impregnation steps which follow. Al-

though nickel was specifically mentioned, any metal capabl e ,of use as an oxide cathodebase metal may be used.

to endure severe vibration and shock without peeling or flaking off. In accordance with my invention, the emissive coating forms as a continuous layer or continuum within the porous matrix which also has acontinuous structure or, continuum of metal throughout its extent. in addition, a higher'ratio of emission per unitarea of cathode is achieved than is possible with conventional cathodes,

thus, smaller cathodes may be utilized for a given rate of emission in accordance with the present. nvention. Further, a low interface resistance between the matrix and the emissive coating is obtained when the elements are made ina'c'cordance withthe present invention.

Therefore, it is an object of the present invention to produce an improved electron emitting cathode-having an electron emitting substance which is tenaciously bonded to thesurface of the matrix, thus producing a more rugged type of cathode structure. I a

lt-is afurther object of the present invention to produce an electronv emitting cathode which is able to endure-mechanical shock and vibration without having the emission coating flake off or peel away from the :It' is still :aifurther object of the present invention .to

provide an electron emitting cathode which has a 'subtion to the insoluble carbonate.

In general, the actual impregnation is carried out in two steps, the first step involving introduction of a soluble alkaline earth metal salt, for example, a barium-strontium salt into the vporous matrix. removed in any convenient manner, for example, by evaporation. After drying, if utilized, a second compound is introduced into the impregnated matrix which precipitates the alkaline earth metal as theinsoluble carbonate in-situ. This second compound may be a soluble metallic carbonate salt such as sodium, potas- Similarly, the oxalate If the hydroxideis utilized as the alkaline earth salt, carbon dioxide gas may be utilized for accomplishing precipita? After precipitation of the carbonate, theby-products are removed and the carbonate converted to the oxide by removal of carbon dioxide gas under influence of heat. After conversion to the oxide, the cathode is ready for activation; Activation may be accomplished by heating the oxide impjregnatedcathode body in the presence of a reducing agent to produce a quantity of free alkaline earth metalin the alkaline earth oxide. For example, activation is accomplished by heating the impregnated cathode body to a temperature of 1000 C. to 1200 'C. for a period suf- The solvent may .be

ficiently long to activate the body, but not for a sufficiently long period to evaporate an appreciable amount of the activating material. For example, I have found that an activating period of from 2 to. 3 minutes is satisfactory. When barium and strontium oxides are present, a portion of these oxides are reduced to the free metal. The reduced portions are chiefly barium; however,s ome strontium is also liberated. Thus, there is created a continuous body or continuum of free alkaline earth metal and the oxide thereof. It is preferable to hold the element out of contact with gases such as air at this timeto prevent recombination of the oxide with carbon dioxide or water, which form undesirable impurities inthe cathode structure.

When a nickel base metal cathode matrix is employed, small amounts of aluminum may be present as a residual or added element. This aluminum acts as a reducing agent in the matrix and has been found satisfactory. I v

In accordance with the preferred modification of the present invention, a porous matrix prepared as above indicated is impregnated with a solution including barium and strontium nitrates. A saturated solution of 50/50 mol ratio is preferred. This primary impregnation may be carried out by immersing the base metal matrix in the solution until the required amount of salt is contained in the matrix, or by placing the solution, for example, by eye-dropper, onto the surface of the raw matrix body. I prefer to utilize a sufficient amount of salt in this step to provide between 1% and by weight of carbonate in the porous matrix. Following this impregnation with the barium-strontium solution, the porous metal bodies are placed in an evacuated chamber and vacuum dried therein. The barium and strontium salt is thus deposited throughout all of the accessible pores of the porous matrix. Following this initial impregnation and drying, a stoichiometric excess of saturated ammonium carbonate solution is introduced into each porous matrix and allowed to pass therethrough; thus leaving a precipitate of insoluble barium-strontium carbonates throughout the pores of the matrix. The solvent, containing the soluble by-product of the precipitation reaction may be removed by leaching for a sufiicient length of time, for example 30 minutes, in distilled water. Sublimation may be employed as an alternativemethod of removal of the byproducts of the reaction when various ammonium salts arepresent. For example, any excess ammonium carbonate, sublimation temperature approximately 58 C.,

may be removed during the drying or sublimation period. The cathode is then activated as previously described.

In a modified form of impregnation, a saturated solution of barium acetate and strontium acetate is introduced into a porous matrix and the impregnation continued until suificient salt is present in the matrix. A saturated solution of sodium carbonate is then introduced into the impregnated matrix until conversion to the carbonate is complete. Excess reactants and by-products of the reaction are then removed by washing or leaching as previously described. The cathode is then ready to be activated, as previously described.

In place of the carbonate, the oxalate may be utilized if desired. The oxalate procedure has the advantage that carbon monoxide, a good reducing agent, is given off as a decomposition product, the reaction being as follows:

heat BaCzO; Ba0+C Oz-l-CO Satisfactory cathodes may be produced in this manner.

Eflicient removal of reactants and by-products is necessary for proper emission of the finished cathode. In

addition to the washing methods previously mentioned, more'elaborate washing methods may be employed, for example, the impregnated matrix may be placed in distilled water for 30 minutes, and thereafter in boiling distilled water for five minutes. The latter method, being much more elaborate, produces a cathode having superior emission characteristics which may be desirable in various operations.

Other soluble barium salts have been found satisfactory for use in the impregnating procedure, such as barium formate. Generally, any soluble barium salt may be utilized for the primary'impregnation; however, it should be noted that the halides generally are not preferred because slight quantities remaining in the matrix after washing and leaching poison the cathode. If efficient washing is accomplished, the use of halides is satisfactory. I prefer to use ammonium carbonate for precipitation since it is relatively easy to remove the ammonium salt formed in the precipitation reaction by either sublimation or washing. If other soluble carbonates, such as sodium or potassium carbonate are used, one must rely solely on leaching or washing to remove the by-product, and this generally is accomplished only after some effort. Their use is otherwise satisfactory.

If desired, the cathode matrix may be first mounted in its assembly, regardless of its complexity, and thereafter impregnated in accordance with the materials and methods set forth above with the additional utilization of an eye-dropper to dispatch the solutions onto the matrix where desired. Such a procedure is readily adaptable to production methods.

Many details of process and procedure may bevaried without departing from the principles of this invention. It is, therefore, not my purpose to limit the scope of this invention other than necessitated by the scope of the ap pended claims.

I claim as my invention:

1. The method of producing an electron emitting cathode body which comprises the steps of impregnating a previously formed porous base metal matrix by exposing said matrix to a solution including a solute of an alkaline earth compound adding a carbonate solution to said matrix, and subsequently converting said alkaline earth carbonate in-situ to the oxide form thereof, and thence activating said impregnated matrix.

2. The method of producing an electron emitting cathode body which comprises the steps of impregnating a porous base metal matrix by means of exposing said matrix to a solution including a solute of a barium-strontium compound adding a carbonate solution to said matrix, and subsequently converting said barium-strontium carbonate in-situ to the oxide form thereof, and thence activating said impregnated matrix.

7 3. The method of producing an electron emitting cathode body which comprises the steps of impregnating a porous base metal matrix by means of exposing said matrix to a solution including a solute of a barium-strontium compound adding a carbonate solution to said matrix, subsequently converting said barium-strontium carbonate in-situ to the oxide form thereof, and thence activating said body by heating to a temperature of from 1000 C. to 1200 C. for a period of from 2 to 3 minutes in the presence of a reducing agent.

4. The method of producing an electron emittingcathode which comprises the steps of impregnating a porous base metal matrix by means of exposing said matrix to an aqueous solution of an alkaline earth compound, converting said compound in-situ to the carbonate, decomposing said carbonate to the oxide, and thence activating said oxide impregnated matrix by heating to a temperature of from l000 C. to 1200 C. in a reducing atmosphere until a quantityof free alkaline earth metal is formed.

5. The method of producing an electron emitting cathode which comprises the steps of impregnating a porous base metal matrix by means of exposing said matrix to an aqueous solution of a barium-strontium compound, converting said compound in-situ to the carbonate, decomposing said carbonate to the oxide, and then activating'said oxideimpregnated matrix by heating to atemperature of from 1000" C. to 1200 C. in a reducing atmosphere until a quantity of free'barium and strontium metal is formed.

6. The method of producing an electron emitting cathode body which comprises the steps of impregnating a porous base metal matrix with a solution of a soluble alkaline earth compound, adding a solution of ammonium carbonate to the impregnated matrix to react with said alkaline earth compound removing the by-products of said reaction from the matrix and thence heating said matrix to a temperature of from about 1000 C. to about 'barium-strontium nitrate, removing the ammonium nitrate from the matrix, and thence activating said impregnated matrix.

8. The method of producing an electron emitting cathode body which comprises the steps of impregnating a porous base metal matrix with a solution of a soluble alkaline earth compound, adding a solution of a salt selected from the class consisting of soluble carbonates and oxalates to the impregnated matrix to react with said alkaline compound removing the by-products of said reaction from the matrix and thence heating said matrix to an activating temperature.

9. The method of producing an electron emitting cathode body which comprises the steps of impregnating a porous base metal matrix with a solution of bariumstrontium nitrate, adding a solution of ammonium oxalate to the impregnated matrix to react with said alkaline earth compound, removing the by-products of the said' reaction from the matrix and thence heating said matrix to an activating temperature.

References Cited in'the file of this patent UNITED STATES PATENTS Kurtz Nov. 13, 1945 Rouse Dec. 21, 1948 Coppola et a1. Nov. 6, 1956 

1. THE METHOD OF PRODUCING AN ELECTRON EMITTING CATHODE BODY WHICH COMPRISES THE STEPS OF IMPREGNATING A PREVIOUSLY FORMED POROUS BASE METAL MATRIX BY EXPOSING SAID MATRIX TO A SOLUTION INCLUDING A SOLUTE OF AN ALKALINE EARTH COMPOUND ADDING A CARBONATE SOLUTION TO SAID MATRIX, AND SUBSEQUENTLY CONVERTING SAID ALKALINE EARTH CARBONATE IN-SITU TO THE OXIDE FORM THEREOF, AND THENCE ACTIVATING SAID IMPREGANTED MATRIX. 